Temper bead (TB) welding is often used as an alternative to PWHT when welding hardenable steels in the fabrication and repair process, commonly employed in the nuclear industry. Historically, qualification of TB welding techniques have employed Charpy V-notch testing to ensure acceptable HAZ fracture toughness; however, ASME Section IX (QW-290) included a provision, in 2004, that allows temper bead qualification through tensile, bend, and peak hardness without impact testing requirements. Previous work shows that for SA-508, a common pressure vessel alloy, which is tempered martensite has the highest toughness of the microstructures present in the heat affected zone.

This work looks to find a quantitative correlation between instrumented indentation, the newly found Hardness Drop Parameter, and critical brittle volume fraction to impact toughness. These three correlations will be the foundation for creating recommendations for the current TB qualification criterion from just hardness indents but based on impact toughness properties. These three correlations will give insight for where to take hardness indents (Instrumented Indentation), give a quantitative estimate for impact toughness from just a hardness indent during qualification and in the field (Hardness Drop Parameter), and finally when a brittle zone is detected how many brittle zones can be present before it becomes detrimental to the mechanical properties of the part (Critical Brittle Volume Fraction). This will give industry partners a specific TB qualification criterion that is simple, safer, and cheaper to implement in the field, and it will shift welding practices toward producing tougher martensitic microstructures as opposed to more brittle bainitic microstructures in the nuclear industry.

      A Frontics AIS 3000 instrumented indenter was used to take indents across TB samples to identify the most brittle point. This would tell industry members where to hardness test in the field and during the qualification process in reference to the fusion line based on welding parameters.

            Over 30 different thermal conditions were created using the Gleeble 3800 with SA508 Gr.3 Cl.2 steel. All samples were austenized at 969⁰C for 5 minutes to keep consistent with previous work and form a homogeneous grain size of 35 microns. Cooling rates from the austenization temperature ranged from 0.5 to 55⁰C/s to create a wide range of HAZ microstructures. A temper then followed for half of the samples at 635⁰C. The untempered simulated materials were then hardness tested followed by polishing, etching with 2% Nital, and point counting to tell the phase fraction of Martensite for each cooling rate. The tempered simulated materials were then hardness tested and impact tested using a Tinius Olsen Model Impact 104 CVN system. The Hardness Drop Parameter was found by taking the difference in hardness before and after tempering.

            After quantitative metallography of the untempered microstructures, the impact energy was also related to the volume fraction of Martensite. This showed a large drop off in impact energy when the Martensite fraction was less than 90 percent. Knowing that the impact energy at -30⁰C of the tempered Bainite and Martensite were 7 and 214 joules respectively, the conservative critical brittle volume fraction for SA508 Gr.3 Cl.2 HAZ microstructures is around 5 percent.

            This work suggests that the ASME maximum hardness threshold be revised due to low hardness not having a direct correlation with impact toughness for structural steels, since tempered Martensite has the highest hardness areas of the tempered HAZ also has the highest impact toughness. The current data proves it is possible that hardness measurements can give a quantitative measurement of impact toughness that can not only be used in TB related projects, but any applications that involve PWHT resulting in savings of millions of dollars in time and resources.


Investigation of Relationship Between Microhardness and Charpy Impact Energy for Temper Bead Welding Qualification \u2013 Part 2

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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